A battery pack is usually composed of a plurality of cells connected in series for supplying electric power to electronic equipments such as an electronic vehicle, portable computer, electronic camera or the like. The battery pack is usually equipped with a voltage detection apparatus that detects the voltage of each cell for capacity calculation and protection of each cell.
FIG. 1 illustrates a prior art voltage detection apparatus 100 for a battery pack in which a plurality of cells are connected in series. The cell voltage detection apparatus 100 is composed of a first input selector 101, a second input selector 103, a detector buffer 105, a data process circuit 110, and a voltage source 111. Generally, an external display unit 113 is connected to the voltage detection apparatus 100 to receive and display the measured cell voltage.
To detect the voltage of each cell in the battery pack, for example, a cell 120, the first input selector 101 selects the positive electrode of the cell 120 and the second input selector 103 selects the negative electrode of the cell 120. Through the first input selector 101 and the second input selector 103, the voltage of the cell 120 is supplied to the detector buffer 105. In the detector buffer 105, the voltage of the cell 120 is subjected to a predetermined calculation to provide an intermediate voltage to the data process circuit 110. The data process circuit 110 processes the intermediate voltage to obtain a voltage value indicative of the cell voltage of the cell 120. The data process circuit 110 may include an analog to digital (A/D) converter 107 and an arithmetic unit 109 as shown in FIG.1 or simply includes a plurality of comparators to determine the voltage value. In FIG.1, the A/D converter 107 converts the intermediate voltage from analog to digital and provides a digital value of the intermediate voltage to the arithmetic unit 109. The arithmetic unit 109 such as a microprocessor processes the supplied digital value in a predetermined manner to acquire the voltage value indicative of the cell voltage of the cell 120. Finally, the display unit 113 can indicate the voltage value on a display screen such as a LCD display panel, plasma display panel, cathode-ray tube (CRT), a fluorescent character display tube or the like.
However, the first and second input selectors 101 and 103 are usually composed of semiconductor switching elements produced using conventional high-voltage complementary metal oxide semiconductor (CMOS) process. Such switching elements impose limitation on application of the voltage detection apparatus 100. The limitation is caused by the fact that the more cells connected in series in the battery pack, the higher break-down voltage required for the switching elements in the first and second input selectors 101 and 103, while such switching elements have a low break-down voltage. Hence taking into account of the low break-down voltage of the switching elements, there has to be a limitation on the cell number to ensure the proper operation of the switching elements. Specially, when the switching elements are constructed of MOSFETs, to ensure the MOSFETs in normal operation, the gate-source voltage of each MOSFET should be always within the safety range, further, the source-bulk voltage of each MOSFET should also be always within the safety range and the body diode of each MOSFET should always be reverse-biased.
With respect to the switching elements that have a high break-down voltage, there may not be such limitation as previously stated. However, the switching elements with the high break-down voltage have to be produced using a more complicated and expensive CMOS process. Consequently, the overall cost of the voltage detection apparatus 100 is increased. Furthermore, compared with the switching elements with the low break-down voltage, the die size of the switching elements with the high break-down voltage usually has to increase a lot to meet the same turn-on resistance requirement, which also imposes an increased cost to the voltage detection apparatus 100. Hence, taking into account of the increased cost and die size, it is not an ideal solution to adopt the switching elements produced using the more complicated and expensive CMOS process to overcome the aforementioned drawbacks.
Accuracy is another aspect that should be taken into account when evaluating a voltage detection apparatus. Typically, inaccuracy is caused by some elements in the voltage detection apparatus. Fox example, in the exemplary voltage detection apparatus 100, a common mode error usually exists in the detector buffer 105 and can degrade the accuracy of the voltage detection. To enhance the accuracy, a common way is to add some supplementary elements or lines, but this will inevitably complicate the circuitry.
It is therefore an object of the present invention to provide a voltage detection apparatus and method that can be realized using the switching elements produced using the high-voltage CMOS process, and at the same time no cost burden is induced and the accuracy is enhanced without complicating the circuitry. It is to such a voltage detection apparatus and method that the present invention is primarily directed.